Hydrocarbon polymers containing aromatic substituents such as polystyrene have poor fire resistance and flame-retarding properties. Such hydrocarbon polymers, when ignited, normally continue burning after ignition and removal of the ignition source. Fire retardancy is the term used to describe the inability of a material to support combustion after removal from the source of ignition. In other words, fire retardant compositions are self extinguishing after removal from a flame used to heat and ignite the same.
It is known that the incorporation of phosphorus atoms as part of the polymeric chain improves the flame retardancy properties of such polymeric materials. Such incorporation of phosphorus is effected by copolymerizing aralkenes with phosphorus compounds containing aliphatic substituents. The polymeric materials resulting from such copolymerization while having fire resistance or flame retardancy properties do not possess the other desirable physical properties such as strength, which is present in hydrocarbon polymers having only carbon-to-carbon bonds in the polymer chain.
It was later discovered, as set forth in U.S. Pat. No. 3,220,989-Rolih, that it is possible to phosphonate aromatic nuclei, having at least one replaceable hydrogen atom on the aromatic nucleus, in hydrocarbon polymers that are substantially free of aliphatic unsaturation derived from the polymerization of polymerizable aralkenes. The nuclear phosphonated hydrocarbon polymers obtained in accordance with that invention possess valuable flame resistance and flame retardant properties, particularly by exhibiting a produced ability to withstand ignition. If ignited by a direct flame application, they do not support combustion, but are immediately self extinguishing on removal of the source of the flame. However, this method results in polymeric chain degradation leading to an undesirably low degree of polymerization of the polymer.
It has been discovered that polystyrene, converted first to a substantially non-crosslinked poly(styrenechloromethyl styrene) copolymer, may be converted further to a phosphorus-containing derivative exhibiting fire retardancy and without a lowered degree of polymerization of the polymer. The product of this invention is characterized by the presence of an intervening methylene group which bonds the phosphorus to the phenyl group of the polystyrene, as opposed to the product of the Rolih patent which features a direct connection of the phosphorus to the phenyl group of the polystyrene.